In the design of most electric or electronic devices consideration is given to the physical size and weight of the object. For many portable devices, there is a desire to optimise the physical parameters. Usually, this means minimising the physical size or weight. Quite often the limiting factor is related to the energy storage device or devices; for example, the battery (e.g. in a mobile telephone) or the capacitor banks (e.g. for a high power laser).
Further design optimisation is achievable by making components multifunctional. The energy source is often one of the largest or heaviest single components in an electric device and is thus one which would benefit most from being multifunctional. One example of multifunctionality is to add mechanical stiffness or load-carrying capacity to an energy storage device. Conventionally structurally strong components are incorporated into the design at the cost of added weight or bulk. As weight is premium in many devices such a component that does not contribute to the load-carrying capacity—it is structurally parasitic.
Conventionally batteries add significant weight and volume without contributing to structural performance; rather, the structural requirements are increased by the need to support and house the batteries. Accordingly attempts have been made to produce multifunctional batteries which contribute to the mechanical strength of the device they supply.
According to one known approach energy storage devices have been fabricated in thin modules that can easily be accommodated within a larger device; this approach has been developed with Li-polymer batteries, the batteries being shaped such that their casing performs a useful structural role. However, the benefits of this approach are limited and these materials are not truly multifunctional in the sense that the active chemicals of the battery are not themselves contributing structurally. Other attempts include autophagous (“self-consuming”) structure-fuel systems and the use of fuel to inflate structural elements. However, these approaches are very limited in application, as the structural properties degrade as the fuel is consumed.
According to other known approaches rechargeable, solid-state thin film composite batteries for integration into structures are used. In this case monofilament reinforcing fibres are coated with conventional solid-state rechargeable batteries in order to provide lightweight structures and ballistic protection with integrated electrical power storage. The materials used for this battery design may be able to bear a portion of the mechanical load, thus providing energy storage that contributes structurally.
In many cases, development of structural batteries has focused on the development of carbon based anodes with both Li+ intercalation ability and reasonable structural properties. A composite battery design based on Lithium ion cells has been developed using a composite stacking approach; a Li+ ion source as anode is separated by a glass fibre fabric layer from a carbon fibre cathode, with a polymer electrolyte matrix. Currently, the prototypes are at an early stage and do not provide net weight benefits. It is not clear how the volume changes associated with intercalation will be accommodated. In addition, a new design of fuel cell based on aluminium foam/carbon fibre sandwich structure, with an internal Nafion electrolyte membrane to separate anode from cathode, is being developed but manifolding and mechanical difficulties are hindering progress. Furthermore, conventional capacitor laminates based on aluminium layers deposited on a variety of insulators (e.g. polycarbonate film, PC/glass, and epoxy/glass) have been fabricated but energy density is currently an order of magnitude below commercial conventional capacitors.
The invention is set out in the claims. The structural energy storage device described herein contributes both energy storage and mechanical strength to a system in which it is incorporated. The mechanical strength is owing to two factors: the design of matted or woven-laminate electrodes based for example on activated carbon fibres and a resin or polymer electrolyte which impregnates the electrodes. The available energy density may be increased by using activated carbon fibres with a high surface area. Power densities greater than that obtained from batteries are realised by capacitance that arises from the electrochemical double-layer at the electrode surfaces.
In overview, the present invention relates to a multifunctional power storage structural device, namely a structural supercapacitor. Mechanical strength is provided by using a composite of woven carbon fibre electrodes and a polymer electrolyte. Unlike, for example, fuel cell solutions, structural components directly provide the energy storage, rather than simply being a small component of an energy system which is mostly liquid fuel. Double-layer supercapacitors also avoid the volume changes and electrode consumption associated with batteries, and, unlike Li-ion systems in particular, have only modest packaging requirements, making them much more adaptable to a range of structural roles.
The most common known form of supercapacitor is based on the electrochemical double layer shown in FIG. 1. In double-layer capacitors, the energy is stored by the accumulation of charge 170 at the boundary between electrode 150 and electrolyte 130. The amount of stored energy is a function of the accessible electrode surface area (which is much greater than the simple geometric area), the size and concentration of the ions dissolved in the polymer electrolyte, and the level of the electrolyte decomposition voltage. Supercapacitors consist of two electrodes 150, a separator 110, and an electrolyte 130. The two electrodes 150 are made of activated carbon, a weak granulated material, providing a high surface area. The electrodes 150 are physically separated by the electrolyte 130, often with an additional separator membrane 110; the electrolyte region 130/110 must be ionically-conducting but electrically insulating. As the dissociation voltage of the organic electrolytes is generally less than 3V, the maximum voltage for a supercapacitor is lower than conventional dielectric capacitors; however, the overall energy and power density is usually higher. Stacks of supercapacitors can be connected in series or parallel. Usually, the electrodes and electrolytes in these systems have no structural performance other than to aid fabrication and provide internal integrity.
Alternative supercapacitor structures are based on the addition of pseudo-capacitative materials, which shifts the performance more towards batteries, by increasing energy density at the expense of (dis)charge rate. They are known as pseudo-capacitors because the electrodes undergo redox reactions whilst electrolyte counter ions accumulate.
Supercapacitors achieve a favourable compromise between two parameters fundamental for energy storage devices: the energy density and the power density. The first parameter defines the amount of energy that can be stored in a given volume or weight (Watt-hours/liter or Watt-hours/kg) and the power density defines the rate at which this energy can be accumulated or discharged (W/kg).
Battery technologies offer high energy density but low power density owing to their reliance on electrochemical reactions to generate charge; typically charge or discharge takes place over a period of a few hours. Capacitors, on the other hand, offer a high power density, typically discharging in a few milliseconds, but a limited energy density. Capacitors provide a rapid means of discharge because all the charge is easily accessible (stored on the surface of conducting plates) but the limited physical size of these plates restricts the maximum charge density they can hold. The total capacitance is also related to the separation of the plates and the dielectric constant of the intervening material but energy storage is still limited to well below that of a battery.
Supercapacitors offer a compromise between batteries and conventional capacitors. They provide a high energy density together with unrivalled power density (typical energy density 6 Wh/kg and power density 0.2-5.0 kW/kg).
According to the present invention, carbon fibres are activated in any appropriate manner, as will be will known to the skilled reader, to provide electrodes 150 with the dual functionality of energy storage and mechanical properties. Referring to the general geometry of FIG. 1, conventional electrodes are replaced by layers of specially activated carbon fibre electrodes 150 and the surface is activated to increase the surface area, whilst not damaging the load-bearing core. The electrodes 150 are separated by an insulating space layer (110), preferably a glass/polymer fibre layer or a porous insulating film. The mesoporosity of the electrodes gives rise to a high contact area between electrolyte and electrode and, thus, the potential for high energy storage.
The electrodes are bonded together by an electrolyte resin which provides simultaneously high ionic conductivity/mobility and good mechanical performance (particularly stiffness). In an embodiment, the electrolytic resin has significant structural capability so as to resist buckling of the fibres in the electrode and provide significant stress transfer. In another embodiment, the polymer resin comprises oxygen-containing groups that coordinate the ions required for the ionic conductivity and cross-linking groups that generate a stiff network. Hence, both structural stiffness and ion mobility are provided.
The embodiment can be further understood with reference to FIG. 2 in which electrode mats 210 encased in a resin which is an ion conductor (not shown) are separated by a glass or polymer fibre mat 230. The electrodes preferably consist of activated structural conducting fibres to form the electrode in a woven 210 or non-woven form (see FIG. 2); the electrodes are preferably based on continuous fibres. Similarly, the glass or polymer fibre mat insulator 230 is woven and sandwiched between the electrode mats 210.
The electrodes may also be based on conductive nanofibres, especially carbon nanotubes or nanofibres. In an embodiment, the conducting fibre is a nanotube or nanofibre mat. In another embodiment, these nano-tubes or -fibres may be combined with conventional conducting fibres; most preferable they are attached to the conventional fibre surface in an approximately radial arrangement.
The electrodes 210 can be formed of unidirectional, woven or NCF continuous fibres, and can be fabricated using standard composite laminate technology known to the skilled reader; for example, liquid resin (such as Vacuum Assisted Resin Transfer Molding, Resin Film Infusion, Rein Transfer Moulding and Resin Infusion Under Flexible Tooling) or pre-preg technologies, as will be familiar to the skilled reader.
According to one embodiment the electrolyte 130 matrix is fabricated as follows: 4 g polyethylene diglycidylether (PEGDGE) mixed with 0.8 g tetrabutyl ammonium hexafluorophosphate (TBAPF6) and stirred, at room temperature, until the salt is completely dissolved. A stoichiometric amount (1 g) of the amine hardener triethyltetraamine (Ciba Gigy) is added and the mixture stirred ready for use.
To fabricate the supercapacitor two layers of woven carbon fibre mats (200 g/m2) are separated by a woven glass fibre mat, in particular samples of carbon fibre mat 10×10 cm are cut sandwiching the glass fibre mat which is cut slightly larger. Prior to impregnation, the mats are rinsed with acetone to remove the epoxy sizing and then undergo atmospheric air plasma treatment. The plasma treatment is conducted at an excitation power of 2 kW at a nozzle to sample distance of 1.5 cm. Both sides of the fibre mat are treated and the mat repeatedly passed under the plasma jet. Each side of the samples is treated for about 5 minutes.
Following plasma treatment the fibre mats are impregnated with the polymer electrolyte 130 from both sides using a small brush and the composite assembled. The samples are cured at room temperature under a small applied pressure (0.5 kg).
It will be seen that the lamination process can be repeated to increase the number of capacitors in series (raising the voltage) as shown in FIG. 3. For example, two laminate supercapacitors 310 and 320 are stacked; each layer containing a conventional carbon matrix 302, a carbon fibre electrolyte matrix 304, a glass mat electrolyte matrix 306, a second carbon fibre electrolyte matrix 304 and a final conventional carbon matrix 302. The outer layers of the composite laminate sequence, the conventional carbon-fibre 302 resin, act as a sealant and contact. Equally they can be constructed of conventional glass/polymer fibre to act, as the seal, with contacts made to the adjacent carbon layers. Ionically-insulating layers may be introduced between each capacitor; for example comprising a layer of conventional carbon-fibre resin composite. The conventional layers 302 additionally improve bending stiffness, and damage tolerance. The fibre orientation in these plies may be of any appropriate lay-up dependent on the structural design considerations.
As a result of the arrangements described above, the provision of mats or woven electrodes 210 and polymer electrolyte 230 provide structural functionality, such that the electrical and mechanical performances of the system approaches those of materials or systems designed solely for either purpose.
In a further embodiment, conducting polymer coatings or metal oxides are added to the electrodes 150 to create ‘pseudocapacitors’ improving the energy storage at the expense of creating a more complex system with slower response times.
In yet a further embodiment, a structural supercapitor is formed based on a radial fibre coating geometry. Such a fibre-sheath design can be applied to the case of a pseudocapacitor design. In order to address the fibres and avoid shorts, in FIG. 4 the carbon fibre electrode 430 is surrounded by a sheath of electrolyte 420 and a conducting outer sheath 410. In another embodiment, there may be additional pseudocapacitive layer between the carbon fibre electrode 430 and the electrolyte 420. In another embodiment, the conducting outer sheath 410 is a pseudocapacitor. A structural resin 440 holds the fibres in place, protects the fibres and transfers the load between them, if necessary. In a further embodiment, the structural resin 440 is electrically conductive; this may be achieved, for example, by adding conductive nanotubes. Four such fibre-sheath supercapacitors are shown in FIG. 4. In FIG. 4 the separate supercapacitors are shown to be isolated by resin; however, preferably, the outer conductive sheath may be shared between two or more fibre supercapacitors. In the limiting case, the outer conductive sheath entirely replaces the structural resin 440. With this system, a packing density greater than 60% could be achieved. The conducting fibres 430 are addressed separately to the outer sheath 410, in particular at the fibre ends.
Supercapacitors can be used directly as a power source but are particularly useful for loading levelling, supplying peak power to complement rechargeable battery system; the combination provides a better lifetime than either component individually.
The laminates can readily be formed into components of almost any shape, using a range of conventional composite processing techniques. The insulating spacer material may exist in particulate, perforated sheet, or fibre form. It is apparent, of course, this stacking approach could be repeated any required number of times.
The potential applications for structural supercapacitors are wide-ranging and numerous; essentially any load-bearing component in a system which requires electrical energy. It would provide considerable weight savings for applications ranging from laptop computers and mobile phones, specialised applications such as down-hole energy supplies for the petrochemical industry, power supplies for emergency equipment and propulsion systems, through to space and military applications. A potentially large market is load levelling in hybrid electric vehicles.